GPS and GIS... whats it all about... can it work in Malaysia?? GIS Maps?? Part 3

So… can we say that GPS is the best way to obtain accurate survey or positional readings on the earth’s surface. Not yet… hold on... there are other factors that govern accuracy of the GPS readings that can alter exact readings.

• Ionosphere and troposphere delays : as you know any wave signal… radio or even microwave faces friction from the atmosphere… the satellite signal can slow or even fade as it reaches the GPS equipment. For this… there are built in components within a GPS unit that calculates the average amount of delay… and it partially corrects the errors.
• Signal multipath: GPS signals can also be reflected off objects as its reaching the GPS receiver…. Tall buildings or large rocks can deflect the signal and this will cause the travel time of the signal to be delayed. As you know this will then cause errors when calculating the distances from satellite position (see Part 2).
• GPS Receiver clock errors : the GPS satellites use atomic clocks … this is different from the GPS receiver’s built in clocks. There will be slight timing errors between both the satellite and the receiver clocks.
• Orbital errors : Since the satellites are thousands of miles away from earths surface… there could be inaccurate readings of the exact satellite’s reported location. This is called ephemeris errors.
• Number of satellites visible : From Part 2…. You can understand that GPS receivers need a minimum of 3 satellites to get positional value. The more satellites that the GPS receiver can detect…. the better accuracy. There are many factors that can block these signals… called interference. This can include… dense trees, buildings, electronic interference,,,, this can cause serious positional errors. GPS units will not work indoors, underwater or underground.
• Selective Availability (SA) : The US Department of defense sometimes adds errors to the signal… this is to prevent its enemies from using the same GPS signals. During the Irag Gulf war… they imposed SA on the satellites.
  

So now you can see there are a number of errors that can cause GPS readings and calculations to be distorted.

Leonardo Da Vinci

I cannot stop admiring the works from this dude. I did mention about him in previous blogs. He potrays so much more than a pretty picture when he paints... his works tell a story, twisted at times... and the longer you stare at them.. the more depth you can get from his subjects. Very similar to Rembrandt, his techniques are so fine and exquisite... both are master storytellers...

GPS and GIS... whats it all about... can it work in Malaysia?? GIS Maps?? Part 2

Well Part 1 covers the basics of GPS. Let’s go into more details of the link with GIS…

OK… can GPS be used in mapping?? Well.. it was initially used for navigation and many mappers/surveyors were skeptical of its accuracy. Mapping accurately used to be a very tiresome process… we used to make maps from ‘control points’ and their position is based on bearings and distances from that ‘control point’. Any object on the earths surface is mapped like this… even today.

These are some of the other mapping / positional systems or practice used:

• Landmarks: Only work in local area. Subject to movement or destruction by environmental factors.
• Dead Reckoning: Very complicated. Accuracy depends on measurement tools which are usually relatively crude. Errors accumulate quickly.
• Celestial: Complicated. Only works at night in good weather. Limited precision.
• OMEGA: Based on relatively few radio direction beacons. Accuracy limited and subject to radio interference.
• LORAN: Limited coverage (mostly coastal). Accuracy variable, affected by geographic situation. Easy to jam or disturb.
• SatNav: Based on low-frequency doppler measurements so it's sensitive to small movements at receiver. Few satellites so updates are infrequent.
  

So how do we get the accurate lat-long position for our GIS data based on this GPS tech??

Step 1 – Lets say based on the timing of the clock between GPS satellite and receiver (see Part 1)… we get a measurement of 20,000 KM. This is from the first satellite. So… now we can make a sphere based on radius of 20,000 KM.

Step 2 - We then get a distance of the second satellite and find out its 21,000 KM away. We create a second sphere based on 21,000 KM radius now.

Step 3 - With the 2 spheres we can position our location where the 2 spheres ‘intersect’.

Step 4 – With the third satellite we get a position of 20,000 KM. We create a third sphere based on 20,000 KM radius now. This will make our position more accurate based on the intersection of this new third sphere.

Step 5 – So by calculating the intersection of 3 satellites and its spheres… we can derive the latitude and longitude of our position in space.

Thats how we can derive exact positions on the earths surface... whoever thought about this is a genius... :D




Claes Oldenburg and Coosje van Bruggen
Typewriter Eraser, Scale X, 1999
National Gallery of Art, Washington
Gift of The Morris and Gwendolyn Cafritz Foundation

GPS and GIS... whats it all about... can it work in Malaysia?? GIS Maps?? Part 1

OK we gotta start with the basics...

How do we calculate the XY position on the earth’s surface?? GPS (Global Positioning System) tech is now widely used to do this.

How did this come about?? GPS is a satellite based navigation system developed by the US Department of Defense. Yes, its usage was mainly in military and spying… focused mainly on military based applications. In the 1980s this changed … civilians can use it too. GPS is good because in works in any weather conditions… anywhere in the world… 24 hours a day.
To get into more technical details… GPS satellites circle the earth twice a day and transmits signal information to earth. There are about 24 satellites orbiting…

So the GPS receiver then receives this information from the satellite… and uses trangulation to calculate the user’s location. Actually what does the satellite transmit?? How is the location calculated??

It works like this… the satellites actually transmit time / clock signals to the earth... there is an internal clock in the GPS satellites. The GPS receiver then compares the time a signal was transmitted by a satellite with the time it was received. The time difference between the signal time and receiver time tells the distance between GPS satellite and receiver. This is for one signal. To actually get an exact location of a point on the ground you need more than one signal… from different satellites … this helps to calculate the measurement in a triangular position. Cool huh…

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

Mr Bush... charity begins at home!!!

Hi there ... another cool site for Maps and Info on Katrina:
http://www.nytimes.com


Well... the whole issue now is to get the water pumps working... and to drain out all the flood water... but to where?? The lake??? River?? Sea?? This is surely gonna create an ecological disaster... The city's waste, toxic material, sewerage, chemicals, rubbish will be polluting for years... if not decades to come. The most important thing during this cleanup is to treat the water before its pumped out... not for drinking ... but at least remove the hazardous content... it may cost more but the benefits are surely greater.

Now among all this... we should not forget the role of the Federal, State and Local governments... this is a superpower we are talking about. To have breakdowns in command and control is something not acceptable... Bush as usual is not a "take charge leader" with brains.. he depends too much on advisors and his senior cabinet members to make decisions... at crisis like this it takes one man strong man to lead... not advisors... its easier to hide behind in a team and point fingers. Clinton is a better example. Mr Bush... charity begins at home!!!

nature's rage... katrina...

For a seemingly innocent and beautiful name like Katrina... her fury is indeed devastating … her face ugly to behold… and strikes unimaginable fear to everyone... "Hell hath no fury like ... "

I watched closely CNN and BBC news updates on New Orleans… paying particular attention to the maps and aerial photos. New Orleans resembled a third world country during floods… similar to the famous Bangladesh floods.. monsoons and floods that lasts for months.

The resemblance is close…

Pre and Post Katrina photos... notice the colour of the water... green and muddy

















Please look at more aerial photos here:
http://www.globalsecurity.org/military/facility/new-orleans-imagery.htm

Also look at the Google Earth images of Katrina here:
http://maps.google.com/

web paradigm .. Malaysia GIS for free... ??

Google Earth has been around for sometime now... this is a clear revolution in GIS and Remote Sensing.... see Rawang (my hometown) on the map.

Using Google Earth software... its so much faster than many conventional IMS servers.... source data is Digital Globe and EarthSat.

Google acquired the company called Keyhole http://www.keyhole.com/body.php?h=about and their Google earth is based on Keyhole technology, they use Keyhole Markup Langueage (KML) http://www.keyhole.com/kml/kml_doc.html for modeling, storing & querying the geographic features


Rawang urban area

Rawang surrounding hills